U.S. patent number 4,786,887 [Application Number 07/080,183] was granted by the patent office on 1988-11-22 for thin-film strain gauge system and method of manufacturing same.
This patent grant is currently assigned to U.S. Philips Corporation. Invention is credited to Udo Bringmann, Olaf H. Dossel, Klaus W. Gerstenberg, Gerhard Kursten, Reiner U. Orlowski, Detlef G. Schon.
United States Patent |
4,786,887 |
Bringmann , et al. |
November 22, 1988 |
Thin-film strain gauge system and method of manufacturing same
Abstract
Thin film strain gauge system consisting of an elastically
deformable flexible metallic substrate on which an electrically
insulating layer of a plasma-polymerized material, in particular of
Si:N:O:C:H-containing compounds and thereon a structured resistance
layer as well as an electrically readily conducting layer having a
structure for the electrical contacting are provided.
Inventors: |
Bringmann; Udo (Halstenbek,
DE), Dossel; Olaf H. (Ellerau, DE),
Gerstenberg; Klaus W. (Halstenbek, DE), Kursten;
Gerhard (Hamburg, DE), Orlowski; Reiner U.
(Quickborn, DE), Schon; Detlef G. (Halstenbek,
DE) |
Assignee: |
U.S. Philips Corporation (New
York, NY)
|
Family
ID: |
6226223 |
Appl.
No.: |
07/080,183 |
Filed: |
July 30, 1987 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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694723 |
Jan 25, 1985 |
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Foreign Application Priority Data
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Jan 30, 1984 [DE] |
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3403042 |
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Current U.S.
Class: |
338/2; 29/621.1;
338/5 |
Current CPC
Class: |
C23C
14/34 (20130101); C23C 16/5096 (20130101); H01J
37/3244 (20130101); H01J 37/34 (20130101); H01L
21/67017 (20130101); Y10T 29/49103 (20150115) |
Current International
Class: |
C23C
16/509 (20060101); C23C 14/34 (20060101); C23C
16/50 (20060101); H01L 21/00 (20060101); H01J
37/32 (20060101); H01J 37/34 (20060101); G01L
001/22 () |
Field of
Search: |
;338/2-5 ;29/61SG |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Goldberg; E. A.
Assistant Examiner: Lateef; Marvin M.
Attorney, Agent or Firm: Spain; Norman N.
Parent Case Text
This is a continuation of application Ser. No. 694,723, filed Jan.
25, 1985, now abandoned.
Claims
What is claimed is:
1. A thin-film strain gauge system comprising an elastically
deformable flexible substrate, an insulating layer present on a
surface of said substrate, a resistance layer on the surface of
said insulating layer away from said substrate and a patterned
electrically conductive layer on the surface of said resistance
layer characterized in that the insulating layer consists of a
plasma polymerized material.
2. A thin-film strain gauge system as claimed in claim 1,
characterized in that the electrically insulating layer (3)
consists of Si:N:O:C:H-containing compounds.
3. A thin-film strain gauge system as claimed in claim 2,
characterized in that the electrically insulating layer (3)
consists of plasma-polymerized oxygen containing silicones or
analogous nitrogen containing silicones.
4. A thin-film strain gauge system as claimed in claim 1,
characterized in that the electrically insulating layer (3) has a
thickness in the range from 0.2 .mu.m to 20 .mu.m.
5. A thin-film strain gauge system as claimed in claim 1,
characterized in that the substrate (1) consists of steel.
6. A thin-film strain gauge system as claimed in claim 1,
characterized in that the substrate (1) consists of a
copper-beryllium alloy.
7. A thin-film strain gauge system as claimed in claim 1,
characterized in that the resistance layer (5) consists of a metal
alloy.
8. A thin-film strain gauge system as claimed in claim 7,
characterized in that the resistance layer (5) consists of a
nickel-chromium alloy.
9. A thin-film strain gauge system as claimed in claim 1,
characterized in that the resistance layer (5) consists of a doped
semiconductor material.
10. A thin-film strain gauge system as claimed in claim 9,
characterized in that the resistance layer consists of silicon
doped with boron or phosphorus.
11. A method of manufacturing a thin-film strain gauge system
comprising:
(a) providing an electrically insulating layer of a
plasma-polymerized material on an elastically deformable, flexible,
substrate by deposition from a gaseous phase which phase comprises
a plasma-polymerizable material;
(b) providing a resistance layer on said electrically insulating
layer,
(c) structuring said resistance layer to form at least one
resistance path and
(d) forming thin layer electrical connections on said structured
resistance layer.
12. The method of claim 11 wherein the substrate is introduced into
a plasma chemical vapor deposition device (PCVD device), monomeric
gases, from which polymerized oxygen containing silicon or
analogous nitrogen containing silicones can be formed by high
frequency excitation, are introduced into said PCVD device said
gases are subjected to said high frequency excitation to thereby
form said polymerized silicone and an insulating layer, formed from
said polymerized silicones is deposited on said substrate.
13. A method as claimed in claim 12, characterized in that
hexamethyldisilazane is introduced as a monomeric process gas.
14. A method as claimed in claim 11, characterized in that the
resistance layer is provided by vapour deposition.
15. A method as claimed in claim 11, characterized in that the
resistance layer is provided by cathodic high frequency
sputtering.
16. A method as claimed in claim 11, characterized in that the
resistance layer is provided by plasma chemical vapour deposition.
Description
BACKGROUND OF THE INVENTION
The invention relates to a thin-film strain gauge system there
having an elastically deformable flexible substrate on which are
provided an electrically insulating layer and thereon a structured
resistance layer as well as an electrically readily conductive
layer having a structure for the electrical contacting, as well as
to a method of manufacturing such a strain gauge system.
For the measurement of the physical quantities mass, power, torque,
acceleration, flow, pressure and pressure differential, measuring
transducers with electric output signals are preferably used.
Measuring transducers based on strain gauges comprise an elastic
element (substrate) which is deformed by the measuring quantity, as
well as a resistor by means of which this deformation is converted
into an electrical signal. Such an electrical signal may then be
amplified and be transmitted over large distances. It may be
introduced into control loops, processed by computers or stored and
it can also be shown easily on displays. The resistor (resistance
layer) can achieve the conversion of the deformation into an
electrical signal through a change of its resistance value.
As a material for the resistance layers there used metal alloys and
semiconductors. For the measurement of the low measuring resistance
variation, for example, four resistance paths formed from the
resistance layer, hereinafter referred to as strain gauges, are
combined to form a symmetrical wheatstone bridge.
The deviation from the bridge equilibrium is proportional to the
elastic deformation of the strain gauge.
Thin-layer strain gauge systems are known in various forms in which
in particular the properties of the electrically insulating layer
between the elastic element and the resistance layer are of
importance. For the electrically insulating layer various materials
have been used which in practice, however, have proved to exhibit
certain disadvantages.
For example, a thin-layer strain gauge system in which inorganic
layers of oxides (Al.sub.2 O.sub.3, MgO or forsterite
2MgO.SiO.sub.2) are manufactured by means of RF cathode sputtering,
electron beam evaporation or with heatable evaporators is known
from DE-OS No. 27 41 055.
Thin-layer strain gauge systems in which inorganic layers of
silicon oxide or silicon nitride are provided by means of plasma
chemical vapour deposition are known from DE-OS No. 30 41 756.
European patent application No. 53 337 discloses thin-layer strain
gauge systems in which the electrically insulating layer consists
of polyimides, polyamide-imides or epoxy-modified polyimides, in
which the layer materials are provided on the substrate as a
solution, are centrifuged and are cured by a tempering
treatment.
Various disadvantages are associated with the known electrically
insulating layers.
Vapour-deposited layers or layers provided by cathode sputtering
result in the coating of only a poor lateral quality. On substrates
having microscopically small unevennesses this leads to
short-circuits between the elastic substrate and the resistance
layer.
The inorganic materials are moreover comparatively brittle and show
haircracks even under small loads which adversely influence the
long-life stability of the strain gauge. Under higher loads it
results in a fracture which leads to interruptions of the
resistance paths.
Although the above-mentioned organic layers may show a high maximum
expansibility, they show poor creeping properties, in particular at
higher temperatures.
BRIEF SUMMARY OF THE INVENTION
It is the object of the invention to improve the thin-film strain
gauge system mentioned in the opening paragraph in such manner that
it does not exhibit the above-mentioned disadvantages, hence to
provide a thin-film strain gauge system whose electrically
insulating layers lead to a good lateral coating, show a high
maximum expansibility and at the same time are stable to above
300.degree. C. and which can compensate for the creeping of the
resilient material (substrate).
According to the invention this object is achieved by having the
electrically insulating layer consist of a plasma-polymerized
material.
According to advantageous further embodiments of the invention the
electrically insulating layer consists plasma-polymerized silicones
or rigid-analogous silicones plasma-polymerized silicones or
silizanes.
A method of manufacturing the thin-film strain gauge system
according to the invention is characterized in that an electrically
insulating layer of a plasma-polymerized material is formed by
deposition from the gaseous phase on an elastically deformable
flexible substrate, after which a resistance layer is provided on
the polymer layer, is then structured to form at least a resistance
track and electrical, thin-layer connections are formed on the
structured resistance layer.
According to advantageous further embodiments of the method
according to the invention the electrically insulating layer is
formed in a plasma chemical vapour deposition device (PCVD device)
in which at least one monomeric gas is introduced from which by
high frequency excitation of the gas phase molecules polymerized
Si:N:O:C:H-containing compounds can be formed which are deposited
on the substrate present in the PCVD device. Advantageously
hexamethyldisilazane is used as a monomeric gas.
The advantages which can be achieved by means of the invention
consist in particular in that the electrically insulating layers of
plasma-polymerized material have a good lateral coating. They show
a good expansibility, are stable up to above 300.degree. C., are
water repellant and chemically resistant. They readily adhere to
all materials from which the elastic substrate is usually
manufactured; moreover, there is also a good bonding to the
overlying resistance layer. The layers can be prepared in any
thickness suitable for the end in view between 0.2 .mu.m and 20
.mu.m. Moreover the possibility is obtained to of compensating for
the creeping of the resilient material (substrate material) by
suitable adjustment of the thickness and the composition of the
layer.
The insulating layers according to the invention show excellent
properties for the intended purposes: the likelihood of a short
circuit to the elastic substrate (resilient member) is very small,
which results in the advantage of a high yield in the manufacture
of the strain gauge systems according to the invention. The
electrically insulating layer according to the invention also shows
a very good expansibility. Typical expansions occurring in pressure
and force transducers are 1.times.10.sup.-3 m/m. In zones of
non-uniform expansion, peaks in the expansion of 2.times.10.sup.-3
m/m can easily occur. If an overload strength should be ensured,
expansions up to 4.times.10.sup.-3 m/m should be withstood without
damage. The present insulation layers satisfy said requirements.
The present electrically insulating layers are stable up to
300.degree. C. This is a particularly important advantage, for, on
the one hand, high process temperatures occur during the
manufacture of the expansion-sensitive resistance layer on the
electrically insulating layer, and, on the other hand, the
adjustment of the desired properties of the resistance layers
usually requires a thermal after-treatment at which temperatures up
to 300.degree. C. are typical. The present electrically insulating
layer is also insensitive with respect to a high relative air
humidity. In conventional strain gauge systems in which a
resistance foil is provided on a synthetic resin support which is
adhered to an elastic deformation carrier, the adhesive and the
carrier material (substrate) consist of organic materials which
expand at a high relative air humidity and so interfere with the
measured signal of the measuring instrument.
The present electrically insulating layers have excellent bonding
properties both to the elastic substrate and to the resistance
layer.
In oxidic insulation layers, for example, the problem exists of the
poor bonding and this difficulty must be overcome by providing
additional layers which serve as bonding intermediate layers,
which, for industrial manufacture, means an additional process step
and hence higher cost.
The present electrically insulating layers also show a particularly
good chemical resistance with respect to agressive liquids and
gases. This is of advantage in regard to the reliability and the
long-life stability of the measuring instrument. In no case may be
electrically insulating layer be attacked by the chemicals used in
the necessary photolithographic structuring process.
A further advantage of the present electrically insulating layers
is that they can be manufactured to be very thin, having
thicknesses of approximately 0.2 .mu.m to 20 .mu.m, in which,
however, they are very free from pin-holes, so that they are very
dense.
Apart from the increase of the process cost which occurs in
providing thick layers in thin-film technology, a good thermal
coupling between the elastic substrate and the resistance layer is
of importance. An immediate bonding is also desired for an optimum
transmission of the expansion profile of the elastic substrate to
the strain gauge formed from the resistance layer.
A further advantage is that the present electrically insulating
layers can be directly adjusted in their creeping properties. The
slow deformation of the elastic substrate (spring member) under
constant load is referred to as creeping. In good resilient
materials the expansion at the surface changes, within five minutes
after a variation of the load, by values between 0.01% and 0.05%
and in this manner causes an error in the pressure and force
measurements, respectively. In bonded strain gauge systems it is
possible to compensate for said creeping, by particularly careful
processing during adhering, for in measuring instruments
manufactured in this manner the adhesive also fatigues and so
produces an opposite creeping.
According to current teaching, such a compensation of the creepage
of the elastic substrate (resilient member) is not possible with
thin-film strain gauge systems (compare W. Ort. Wagen und Dosieren,
1979, No. 3, p. 86). However, it has surprisingly been found that
with the thin-film strain gauge systems manufactured according to
the invention it is possible, in contrast with the current
teaching, to compensate for the creeping of the material of the
elastic substrate. Different materials for the elastic substrates
show different values both in the final value after 5 minutes and
in the time variation of the creeping. By directed adjustment of
the thickness and the proparation parameters the opposite creeping
behaviour of the electrically insulating layers can be accurately
adjusted so that the creepage error of the strain gauge system
remains below 0.01% on typical materials for the elastic substrate,
for example, steel. For this purpose reference is made to the data
of the example.
BRIEF DESCRIPTION OF THE DRAWING
FIG. 1 is a cross-sectional view of a thin film strain gauge system
of the invention and
FIG. 2 is a diagram of an RF-diode arrangement for manufacturing an
electrically insulating layer of the invention.
DETAILED DESCRIPTION OF THE INVENTION
An embodiment of the invention will be described in greater detail
with reference to the drawings in which
FIG. 1 is a diagrammatic sectional view through a thin film strain
gauge system according to the invention.
FIG. 2 shows a diagram of an RF-diode arrangement for the
manufacture of an electrically insulating layer according to the
invention.
FIG. 1 shows diagrammatically the construction of a layer of a
thin-film strain gauge system. An approximately 3 .mu.m thick layer
of plasma-polymerized material is provided as an electrically
insulating layer 3 on an elastically deformable, flexible substrate
1, for example of steel, and hereon an approximately 0.3 .mu.m
thick resistance layer 5, for example of a nickel-chromium alloy is
provided by means of cathode sputtering. An approximately 1 .mu.m
thick readily conductive layer 7, for example of gold, is then
provided for the manufacture of contrast pads and internal
connections, for example, by cathode sputtering.
As process gases for the manufacture of the plasma-polymerized
electrically insulating layer 3 hexamethyldisilazan was used in a
plasma chemical vapour deposition device 9 (compare FIG. 2). The
introduced gas is split in the high-frequency plasma produced in
the PCVD device 9. On a substrate provided in the PCVD device 9 on
one of the electrodes 17, 19, three-dimensionally crosslinked
macromolecules with Si-O-Si-bonds or Si-N-Si-bonds are formed by
polymerization reactions.
A layer manufactured with hexamethyldisilazan as a
silicon-containing process gas was prepared under the following
process conditions:
The PCVD device 9 was first evacuated with a vacuum pump 15 to a
pressure of a few 10.sup.-4 Pa. Hexamethyldisilazan was supplied as
a process gas via one of the inlets 13 at a pressure of 0.02 Pa.
The electrode spacing was 50 mm. A 27 MHz HF generator was used.
The potential adjusting at the RF electrode 17 on which at least
one substrate 11 to be coated was provided was 250V direct voltage.
After 60 minutes' deposition a 3 .mu.m thick polymer layer 3 was
obtained on the substrate (s) 11.
This example describes the excitation of the gas phase molecules by
a high frequency voltage. Besides the excitation by means of the
diode arrangement shown in FIG. 2 an inductive or a capacitive
excitation of the gas phase molecules is in principle also
possible.
After depositing the electrically insulating layer 3 the resistance
layer 5 is provided. In addition to the already mentioned alloys
NiCr or PtW, CrSi or doped semiconductors are also suitable as
materials for said layers. CrNi was used for the said examples.
Resistance layers of all the said materials are very stable and
comparatively high-ohmic. The resistance layer can be provided by
any method of thin-layer technology known to those skilled in the
art. According to this example an RF-cathode sputtering process was
used.
The layer 7 of electrically readily conducting material, for
example gold, present on the resistance layer 5 for the connection
contacting is also manufactured by RF cathode sputtering.
In two photolithographic process steps the contact pads of the gold
layer 7 as well as a Wheatstone bridge structure of the resistance
layer 5 are manufactured. The required etching steps may be carried
out wet chemically or by ion bombardment (back sputtering).
The electrical connections of the resistance bridge are formed by
four gold wires which are connected on the contact pads formed from
the layer 7 by means of thermocompression.
The temperature coefficient of the electric resistance (TC.sub.R)
of the strain gauges obtained from the resistance layer normally
has a low, negative value. By a thermal after-treatment (tempering)
at approximately 300.degree. C., however, it can be adjusted.
Depending on the duration of tempering the resistance values become
smaller, the TC.sub.R on the contrary becomes higher and even
positive. By choosing a suitable tempering period the influence of
the temperature on the resistance can be kept neglibly small. These
measures are known to shose skilled in the art.
As the last step in the process it is recommendable to provide a
passivating layer on the strain gauge system which may not
adversely influence the elastic behaviour of the flexible
substrate; advantageously its layer thickness is limited to
approximately 10 .mu.m.
Technical data of thin-film strain gauge systems manufactured
according to the invention: Substrate: noble steel, polished
Insulating layer: polymer layer, layer thickness 3 .mu.m
Resistance layer: NiCr, layer thickness 0.3 .mu.m
Sheet resistance: 4 .OMEGA./.quadrature.
Resistance of an individual resistance path: 130.OMEGA.
Temperature coefficient of an individual resistance path: +10
ppm/K
Behaviour of the relative resistance variation to the expansion
(expansion sensitivity): 2.2
Creeping (creeping error): <0.01%
Resistance of the insulating layer: >10.sup.11 .OMEGA..cm
Maximum permissible expansion: 2 to 10.times.10.sup.-3 m/m
Pinhole density: 2/cm.sup.2
Breakdown field strength: 150V/.mu.m.
* * * * *